1. Field of the Invention
This invention relates generally to aerodynamic surfaces and, more particularly, to improved constructions and control schemes for such aerodynamic surfaces which provide for aerodynamic control and for significant reductions in noise in the case of rotor blades.
2. Description of Related Art
The prior art continues to encounter problems with providing controllable aerodynamic surfaces on rotor blades, wings, engine inlets, helicopter fuselages, and nozzles. Movable control surfaces placed on these aerodynamic surfaces have included flaps, slots, spoilers, ailerons, elevators, and rudders. Although these control surfaces can mechanically alter the geometry of the original aerodynamic device, they are limited in ability to respond quickly and efficiently. Prior art mechanical control surfaces can add mechanical complexity to the aircraft, can compromise structural integrity, can complicate manufacturing and can compromise radar detectability.
Conventional helicopters in descent flight conditions can also generate an impulsive noise signature which is commonly referred to as blade-vortex interaction (BVI) noise or xe2x80x9cblade slap.xe2x80x9d BVI noise is generated by blade tip vortices, that interact with the rotor blades. Unfortunately, BVI noise is typically within a frequency range which is highly important to human subjective response. Additionally, it is easily detected electronically at large distances, thus increasing the vulnerability of military rotorcraft. Consequently, a reduction in the BVI noise intensity and changes in the noise signature, using active and/or passive noise control techniques, are desirable to the rotorcraft industry, which is challenged by today""s stringent military and civilian acoustic regulations.
In addition to being detectible by BVI noise, an aircraft can also be detected by its infrared light signature. Accordingly, a means for efficiently and effectively reducing the infrared light signature of an aircraft is needed.
Three possible measures may be taken to reduce BVI noise. Namely, the tip vortex strength may be weakened, the separation distance between the blade and the tip vortex may be increased, and/or the blade geometry may be altered. The result of these measures is a decrease in the strength of the interaction between the rotor blade and the tip vortices. Existing devices which have been used for reducing BVI noise include the use of a blade mounted trailing edge flap which seeks to change the strength of the tip vortex and hence the intensity of BVI and the use of Higher Harmonic root pitch control (HHC), which seeks to change the blade vortex separation distance, and thus the local aerodynamic conditions, through blade pitch changes.
Other control means concentrate primarily on reducing the strength of the tip vortex through blade tip geometric modifications. Typical examples are the use of leading and trailing edge sweep, the use of blade anhedral, spoilers, and the use of a subwing concept. All of these examples, excluding HHC, may be classified as passive control techniques. An example of another active control technique would be the use of tip air mass injection, which again has the purpose of weakening the blade tip vortices. Tip air mass injection involves introducing a high energy air jet at the tip of the blade, aimed at the center of the tip vortex with the sole purpose of diffusing or weakening its strength.
Each of the prior art solutions to BVI noise has been at least partially unsuccessful, either because of ineffectiveness or because of the solution""s detrimental side effects with respect to the flight characteristics and efficiency of the rotorcraft. For example, HHC methods change the aerodynamic conditions along the entire blade in order to reduce BVI noise, due to the change in blade pitch. Passive BVI noise control methods are not adaptable to other BVI and non-BVI conditions throughout the flight regime that are associated with changes in descent rate and forward flight speed. Additionally, most of the passive prior art solutions to the BVI problem are deployed at all times, whether or not needed, often degrading flight performance unnecessarily.
Another longstanding problem with conventional aircraft involves vibrations of the rotor hub and general inefficiencies of the rotor blade. Rotor blades which route jet engine exhaust to the tips thereof require a significant amount of jet exhaust, which can reduce an available amount of forward thrust of the resulting aircraft. Rotor blades of tiltrotor aircraft commonly introduce problems of download and fountain flow into the resulting system. Download and fountain flow conditions can be generally attributed to the downwardly directed air from the rotor blades in hover mode being directed onto the upper surfaces of the wings of the tiltrotor aircraft.
This invention addresses the aforementioned problems by providing an active control device that has a number of advantages over prior art approaches. A porous surface on an aircraft structure driven with oscillating positive and negative pressures is used as an active control device for attenuating negative aerodynamic interactions. The porous surfaces can be driven with positive and negative pressures either continuously or when predetermined flight conditions are present. The porous surfaces can be used on rotor blades to reduce BVI noise in descent flight conditions.
The porous surfaces of the present invention can be configured on rotor blades for affecting blade variable twist in accordance with various flight conditions, and can further be incorporated for reducing rotor hub vibrations as well. Porous surfaces placed on aerodynamic surfaces below the rotor blades of a tiltrotor aircraft can attenuate or eliminate download and fountain flow conditions. When placed on the trailing edges of a tip jet-exhaust driven rotor blade, the porous surfaces of the present invention can supplement the tip jet momentum of the exhaust to thereby reduce an amount of exhaust needed to drive the rotor blade. The porous surfaces can be used on other aircraft structures, such as wings, engine inlets, engine exhausts, blunt surfaces and nozzles.
The present invention, together with additional features and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying illustrative drawings.